This invention relates to a variable length frame exchanging method, a variable length frame exchanging interface apparatus and a call acceptance determination system adapted for a fixed length cell handing exchange in which a terminal for use with a data communication system by a variable length frame is accommodated.
In recent years, as the introduction of the ISDN (Integrated Services Digital Network) proceeds, the utilization of public networks is complicated such that a public network, which has offered only telephone services, can now offer such various communication services as data communications, facsimile communications and packet communications at home of each of subscribers. Further, as the amount of communications increases, not only speech but also video signals and so forth can be transmitted. In this manner, various information communications are available nowadays. Particularly it is advantageous in terms of the cost and/or the communication speed to communicate data by way of an ISDN.
Meanwhile, the CCITT recommends a data communication method by a variable length frame including logic channel information, for example, a frame relay, for data communications. The frame relay is adapted to a common packet exchanging system and is expected as a technique which meets the requirement for a high speed packet service. In the frame relay technique, no protocol for a layer higher than the layer 2 is defined and the network does not participate in the protocol since frame multiplexing is performed in the data link layer, and accordingly, hardware matching is easy comparing with a packet exchanging system based on the X25 which is a protocol for the layer 2 and an increase in throughput can be realized.
Further, in popular packet exchanging systems, repeat control based on a protocol of a LAP-B is performed between a terminal and a network (exchange) or between different exchanges. With a frame relay, however, repeat control is not performed between a terminal and a network, and consequently, high speed transmission can be achieved together with an additional reason that no controlling processing for the repeat control is required.
By the way, in recent years, an ATM (Asynchronous Transfer Mode) exchange technique has been consented as a next generation exchange system by the CCITT, and investigations have been and are being directed actively toward the ATM exchange technique as a technique which realizes a broad band ISDN (B-ISDN). With an ATM exchange (fixed length cell handling exchange), user information is exchanged in the form of a packet of a fixed length called cell which includes united contents (data, sound, images and so forth) of information. As development of such ATM exchange technique proceeds, it is desired that also the service for accommodating and connecting a terminal (hereinafter referred to as frame relay terminal) for a frame relay described above into and to an ATM exchange network can be offered as one of services of the ATM exchange network.
An example for reference of a system wherein an ISDN frame relay is used to interconnect a plurality of LANs (Local Area Networks) is illustrated in FIG. 44. Referring to FIG. 44, a plurality of personal computers (frame relay terminals) 11 to 16 are shown, and a LAN 21 is constituted from the personal computers 11 and 12; another LAN 22 is constituted from the personal computers 13 and 14; and a further LAN 23 is constituted from the personal computers 15 and 16. The LANs 21 to 23 are connected to exchange offices 51 to 53 by way of ISDN interfaces 31 to 33 and circuit terminating equipments (NT) 41 to 43, respectively, so that they are connected to each other by way of a public network (ISDN) 60.
In such a system as described above, establishment of a transmission channel is performed using a call control protocol (I.451), but a transmission channel may otherwise be established in response to a command from a service person. Meanwhile, a LAP-F (Link Access Protocol-Frame Relay) is used for frame transmission (popular packet transmission). Data transmission is thus allowed between arbitrary terminals (personal computers 11 to 16). In particular, each of the personal computers 11 to 16 can communicate with an arbitrary one of the other personal computers 11 to 16 by performing data communication by way of a logical link (identified with a DLCI (Data Link Connection Identifier) in the signal) number of the LAP-F applied on the communication channel.
In this instance, the transmission channel between any one of the LANs 21 to 23 and a corresponding one of the exchange offices 51 to 53 can have an arbitrary line speed, and each of the LANs 21 to 23 can select a desired line speed. For example, a PCM circuit (24 channels or 30 channels) is used for the interface with each subscriber, and arbitrary channels desired by the subscribers can be combined to determine a line speed. In short, a subscriber can select a line speed from among up to 64.times.24 channels (or 64.times.30 channels) to the utmost.
However, where the network shown in FIG. 44 is a broad band ISDN which makes use of an ATM exchange, a frame relay has a subject to be solved in that, since it involves a variable frame length similarly to that of the popular packet exchange system X25, it cannot achieve high speed exchanging processing comparing with switching which employs a packet (cells) of a fixed length such as in an ATM.
Further, the CCITT does not involve provisions for the inside of a network (that is, for the inside of an exchange) when a frame relay terminal is accommodated into an ATM exchange, and the method of realizing such accommodation depends upon the maker of the system. For example, in the system shown in FIG. 44, a communication channel is connected between arbitrary ones of the subscribers (personal computers 11 to 16), and when any of the subscribers tries to transmit data to the other party, a function of routing to the other party designated by a DLCI number is required.
Meanwhile, in order to accommodate a frame relay terminal into an ATM exchange and set a path, a system has been proposed wherein acceptance of a call is determined from a peak value (a value reported from the user) of a band used between terminals between which communications are to be performed and a band for the path is allocated in accordance with the determination. Accordingly, when a user of a frame relay terminal makes a request to set a path of a band higher than a physical line speed (physical maximum speed), the path cannot be set nor connected.
However, a frame relay service allows, in most cases, the contract of generally 200 to 300% of the physical line speed, and also when a frame relay service is offered with an ATM exchange, the conditions must be the same as those in the existing service. Therefore, when a frame relay terminal is to be accommodated into an ATM exchange, it is desired to allow determination of acceptance of a call from a frame relay terminal while adopting the same conditions as those of the existing frame relay service.